1732 zyxwvutsrqpon Organometallics 1989, 8, 1732-1736 stantial 7r,v4-character, more so than is present in the (v5-C5H6),M(diene)(M zyxwvutsrq = Zr, Hf) complexes. This in turn suggests that the M(I1) oxidation state contributes sig- nificantly to the bonding scheme of the derivatives zyxwvu M- (v4-C4H6)R[ N(SiMe2CH2PR2)2]. These new group zyxwvutsr 4 butadiene complexes display flux- ional behavior that is best described as diene rotation rather than the envelope-flipping mechanism normally associated with early metal diene derivatives. That this fluxional process is more akin to that found for an elec- tron-rich, late-transition-metal complex provides further support for the proposal that phosphine donors can sta- bilize lower oxidation states of the early transition metals.31 (31) For recent examples of this strategy, zyxwvutsrqpo see: (a) Wielstra, Y.; Gam- barotta, s.; Roedelof, J. B.; Chiang, M. Y. Organometallics 1988, 7, 2177. (b) Wielstra, Y.; Gambarotta, S.; Chiang, M. Y. Organometallics 1988, 7, 1866. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Acknowledgment. Financial support was provided by NSERC of Canada. Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this work. We are also grateful to Professor James Trotter for the use of his diffractometer and structure solving programs. Registry No. la, 94372-15-3; l b , 98758-74-8; IC,94372-17-5; Id, 94372-16-4; 2a, 120638-28-0; 2b, 113686-58-1; 2c, 120638-29-1; 2d, 113686-59-2; 3a, 120638-30-4; 3b, 120638-31-5; 4a, 120638-32-6; 4b, 120665-67-0; Mg'C4&, 60300-64-3; LiPh, 591-51-5;LiCH2CMe,, Supplementary Material Available: Tables of calculated hydrogen atom parameters, anisotropic thermal parameters, and bond lengths and angles involving refined hydrogen atoms (Tables S1-S4) (5 pages);listings of structure factors (Tables S5 and S6) (50 pages). Ordering information is given on any current masthead page. 7412-67-1. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLK A Survey of Catalytic Activity of $-Cyclopentadienyl Complexes of Groups 4-6 and Uranium and Thorium for the Dehydrocoupling of Phenylsilane Clare Aitken, Jean-Pierre Barry, Francois Gauvin, John F. Harrod," Abdul Malek, and Denis Rousseau zyxwv Department of Chemistry, McGill University, Montreal, zyxwv PQ, Canada, H3A 2K6 Received December zyxwvut 2, 1988 The effectiveness of a variety of metallocene, metallocene alkyl, and metallocene hydride complexes of group 4-6 metals and of U and Th as catalysts for the dehydrocoupling of phenylsilane was studied. Among the d-block complexes only those of Ti and Zr showed high activity for the production of higher oligomers (degree of polymerization 10-20). Vanadocene is an effective catalyst for the synthesis of 1,2-diphenyldisilane and 1,2,3-triphenyltrisilane from phenylsilane but under much more forcing condition than necessary with the group 4 catalysts. The metallocene hydrides of Mo, W, Nb, and Ta react slowly with phenylsilane to give relatively stable metallocene silyl hydrides or other decomposition products that show no catalytic activity. The synthesis and characterization of a number of new hydridosilylmolybdenocene complexes are described. Although the organouranium and -thorium complexes show some catalytic activity for dehydrocoupling of phenylsilane in ether solution, their instability and the complexity of the chemistry involved mitigates against their usefulness as practical catalysts. An explanation of the unique activity of Ti and Zr complexes is proposed that invokes the necessity for an empty nonbonding orbital on the metal to facilitate an a-hydrogen elimination from coordinated silyl ligand. Introduction A number of promising applications of polysilanes has recently aroused considerable interest in their synthesis and the study of their physical and chemical properties.' The only known route to poly(organosi1anes) of high mo- lecular weight is the condensation of diorganodihalosilanes through reaction with a group 1 metal (the Wurtz-Fittig reacti~n).~,~ Although this method is highly versatile and can give linear polymers with molecular weights up to ca. lo6, it suffers from the fact that it is difficult to control and usually gives large amounts of cyclic oligomers and/or a low molecular weight fraction. The discovery of catalysts for the facile dehydrocoupling of primary organosilanes aroused hopes that such reactions might provide another, and perhaps superior, synthesis of poly~ilanes.~+ The first class of catalysts explored by us were compounds of the type Cp2MR2, where Cp is an (1) West, R.; Maxka, J. In Inorganic and Organometallic Polymers; Zeldin, M.; Wynne, K. J.; Allcock, H. R., Eds.; ACS Symposium Series 360; American Chemical Society: Washington, DC, 1988; Chapter 2 and references therein. (2) Burkhard, C. A. J. Am. Chem. SOC. 1949, 71, 963. Kumada, M.; Tamao, K. Ado. Organomet. Chem. 1968, 6, 19. West, R. Pure Appl. Chem. 1982,54,1041. Hengge, E. Top. Curr. Chem. 1974,51,1. Yajima, S.; Hayashi, J.; Omori, M. Chem. Lett. 1975, 931. (3) West, R.; David, L. D.; Djurovic, P. I.; Stearley, K. L.; Srinivasan, K. S. V.; Yu, H. J. Am. Chem. SOC. 1981, 103, 7352. Trujillo, R. E. J. Organomet. Chem. 1980, 198, C27. Wesson, J. P.; Williams, T. C. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 2833. (4) (a) Aitken, C. T.; Harrod, J. F.; Samuel, E. J. Organomet. Chem. 1985,279, C11. (b) Aitken, C. T.; Harrod, J. F.; Samuel, E. J. Am. Chem. SOC. 1986, 108, 4059. (c) Aitken, C. T.; Samuel, E.; Harrod, J. F. Can. J. Chem. 1986, 64, 1677. Harrod, J. F.; Yun, S. S. Organometallics 1987, 6, 1381. (5) Harrod, J. F. Inorganic and Organometallic Polymers; Zeldin, M., Wynne, K. J., Allcock, H. R., Eds.; ACS Symposium Series 360; American Chemical Society: Washington, DC, 1988; Chapter 7. Harrod, J. F. The Design, Activation and Transformation of Organometallics into Com- mon and Exotic Materials; Laine, R. M., Ed.; NATO AS1 Series E, 141; Martinus Nijhoff: Amsterdam, 1988; p 103. (6) Aitken, C. T.; Harrod, J. F.; Gill, U. S. Can. J. Chem. 1987, 65, 1804. 0276-7333/89/2308-1732$01.50/0 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 0 1989 American Chemical Society